1. UART and UART Driver B. Ramamurthy

2. UART: Universal Asynchronous Receiver/Transmitter device Device connected to the system bus; it takes parallel data converts into RS232C serial format and transmits it; and receives data in serial format and converts it to parallel format for CPU bus -- parallel byte is converted into start bit, 8 bits, parity bit and stop bit serial form and vice versa Asynchronous transmission allows data to be transmitted without the sender having to send a clock signal to the receiver. Instead, the sender and receiver must agree on timing parameters in advance and special bits are added to each word which are used to synchronize the sending and receiving units. -- That’s why you set the baud rate in your xinu environment UART

3. Lets study the layout and operation of 16550 UART chip. Page 1: The features Page 2: Basic chip configuration Page 6 …: RD and WR cycle timing Page 11: Pin descriptions Page 14: Register summary Page 15…: Register explanation 16550 UART Chip

4. Serial communication standard. See the DE-9 pin configuration: G (5), TxD (3), RxD(2), DTR(4), DSR(6), RTS(7), CTS(8), DCD(1), RI (9) RS 232C Standard

5. xinu_mips-1.0.2/include/uart.h xinu_mips-1.0.2/uart/* Map all that you learned in the last few slides to the code in uart.h and the driver implementation in uart directory XINU UART Driver

6. http://www.freebsd.org/doc/en/articles/serial-uart/ http://cache.national.com/ds/NS/NS16C552.pdf http://xinu.mscs.mu.edu/National_Semiconductor_16550_UART#Physical_UART http://en.wikipedia.org/wiki/RS-232 Linux device-driver: http://www.networkcomputing.com/unixworld/tutorial/010/010.txt.html References

7. Device driver context User process Operating System: dev/xyz Device driver Physical Device hardware

8. UART Driver in EXINU • General device driver related files: device.h, devtable.c • Uart files: uart.h defining the physical features of the uart • All the files in the uart directory that implement the operations related to the uart. • uartControl.cuartInit.cuartIntr.c • uartPutChar.cuartWrite.cuartGetChar.cuartRead.c

9. Device Drivers • On board devices are called internal peripherals and one outside are called external peripherals • UART Chip (internal) • TTY (external) • UART transceiverRS232D-9 connector  laptop serial socket • WRT54GL board and modifications

10. Serial Port Block Diagram (general) Processor RS-232 Transceiver COM1 DB-9 connector UART Peripheral

11. Our Setup WRT54GL Processor: BCM5353 RS-232 Transceiver AMD202 COM1 DB-9 connector UART Peripheral TI 16552

12. Device drivers • Embedded processor interacts with a peripheral device through a set of control and status registers. • Registers are part of the peripheral device. • Registers within a serial controller are different from those in a timer. • These devices are located in the memory space of the processor or I/O space of the processor-- two types: memory-mapped or I/O mapped respectively.

13. Device driver (contd.) • The keyword volatile should be used when declaring pointers to device drivers. • Our text goes on to discuss bit manipulation: please read it) • Bit patterns for testing, setting, clearing, toggling, shifting bits, bitmasks, and bitfields. • Struct overlays: • In embedded systems featuring memory mapped IO devices, it is common to overlay a C struct on to each peripheral’s control and status registers. • This will provide the offsets for the various registers from the base address of the device.

14. Device Driver Philosophy • Hide the hardware completely: hardware abstraction • If the device generates any interrupts include interrupt controllers. • Device driver presents a generic interface for applications at higher level to access the devices: device.h • Device drivers in embedded systems are different from general purpose operating systems: See diagram in slide #7 • Applications in general purpose systems accesses OS (Operating Systems) which in turn accesses device drivers. • Applications in embedded systems can directly access device drivers.

15. General Purpose OS vs. Embedded System Application process Application process Operating System: dev/xyz Device driver Device driver Physical Device hardware Physical Device hardware

16. Device Driver development steps • An interface to the control and status registers. • Variables to track the current state of the physical and logical devices -- Major and minor device number, device name • A routine to initialize the hardware to known state • An API for users of the device driver -- Read, write, seek • Interrupts service routines

17. Example: A serial device driver • Now lets look at the UARTdriver of the embedded xinu and WRT54GL. • Study the tty driver that is a logical device that is layered on top of the UART driver. • Framebuffer is an example of logical device driver.

18. Summary • We studied the design and development of device drivers. • We analyzed the code for a sample UART driver. • Read the UART device driver exinu and wrt54gl.